Aspnes and the present inventor disclosed an ellipsometer in U.S. Pat. No. 5,091,320, and incorporated herein by reference. This highly sensitive ellipsometer allows the characterization of the composition of a semiconductive thin film as it is being grown in a closed deposition chamber. As illustrated in the schematic illustration of FIG. 1, the thin film is grown on a substrate 10, usually a semiconductor wafer, held on a substrate holder 12 located inside the enclosed growth chamber. If the growth process is molecular beam epitaxy (MBE) or organo-metallic molecular beam epitaxy (OMMBE), the growth chamber is a vacuum chamber 14 held at fractions of a torr. Two viewing ports 16 and 18 are disposed at about 70.degree. on either side of the surface normal 20 of the substrate holder 12. Low-strain optical windows 22 in the viewing ports 16 and 18 provide optical access to the growing thin film. A viewing port 24 with its own window is usually located on the surface normal 20. The ellipsometer of Aspnes and Quinn includes a light source 26 outputting a beam along an incident axis 28, an entrance iris 30, a rotatable input polarizer 32, a rotatable output polarizer 34 and an exit iris 35 disposed on a reflection axis 36, and an optical detector 38. A computer 40 receives the measured optical intensities and controls the polarizers 32 and 34. They chose to use a wide band Xe arc lamp as the light source 26 and include a monochromator just before the optical detector 38. More details may be found in their patent. The ellipsometer provides an incident beam of a predetermined polarization state extending through the window of the input port 22 and striking the substrate 10 at an oblique angle .phi..sub.i away from the surface normal. The incident radiation interacts with the top micrometer or so of material on the substrate 10 (i.e., the growing thin film) and is reflected in a changed polarization state along the reflection axis 36 extending through the output port 18. The reflection axis 36 is angularly displaced from the normal 20 by .phi..sub.0. For specular reflection, EQU .phi..sub.i =.phi..sub.0. (1)
The ellipsometer of Aspnes and Quinn has been used to control the growth of thin films of ternary semiconductors, e.g. Al.sub.x Ga.sub.1-x As. It is highly precise, capable of controlling variations of the alloying percentage x to less than 0.1%
However, its accuracy has been substantially less. Accuracy depends on the absolute value of the alloying percentage x, not just its variation. The accuracy has been limited by the lack an exact numerical value of the incident angle .phi..sub.i, which is used in reducing the experimentally derived ellipsometric data to the complex refractive indices or dielectric functions of the material being probed by the ellipsometer. In the past, the incident angle .phi..sub.i was determined by triangulation. However, the sample holder 12 is within the vacuum chamber 14 having very limited optical access, and its angular position cannot usually be moved. As a result, triangulation becomes difficult. Another commonly used technique involves auto-collimation in which the sample is replaced by a mirror mounted on a precision rotatable stage. The stage or light source is then moved until the reflected beam coincides with the incident beam. Thereafter, the stage is rotated through a precisely measured angle to its operational position. Reflective auto-collimation is not useful with growth chambers because the substrate holder is designed for considerations inconsistent with a precision rotatable stage. Another indirect technique involves mounting a reference sample of known refractive indices and performing ellipsometry upon it. The value of .phi..sub.i which best reduces the ellipsometric data to the known (or assumed) dielectric function of the sample is taken as the incident angle. However, this technique requires that the actual surface composition of the reference sample be precisely known and be constant. Therefore, the accuracy of the indirect approach depends on the unknown reproducibility of the reference sample.